The present invention relates to an image forming apparatus of the type forming a toner image on an image carrier, transferring the toner image from the image carrier to a recording medium, and cleaning the image carrier with a cleaning device after the image transfer.
An image forming apparatus of the type described is implemented as, e.g., an electrophotographic copier, a printer, a facsimile apparatus or a multiplex machine operable as at least two of them. Further, the image forming apparatus may be implemented as a so-called tandem image forming apparatus including a plurality of image carriers. The image carriers are sequentially arranged in a direction in which a paper sheet or similar recording medium is conveyed (direction of paper conveyance hereinafter). A plurality of developing devices each face a particular image carrier and form a toner image of particular color on the image carrier. Toner images so formed on the image carriers are sequentially transferred to a paper sheet one above the other. A particular cleaning device is assigned to each image carrier in order to remove toner left on the image carrier after the image transfer. Toner collected by the individual cleaning device should preferably be returned to the associated developing device and again used to form a toner image. This successfully saves toner and therefore promotes the efficient use of limited resources. Let such a system be referred to as a toner recycling system.
However, the tandem image forming apparatus using the toner recycling system has the following problem left unsolved. After a toner image has been transferred from one image carrier positioned at the upstream side in the direction of paper conveyance to a paper sheet, toner forming the toner image is transferred from the paper sheet to the next image carrier positioned at the downstream side although in a small amount. The cleaning device assigned to the downstream image carrier collects the extraneous toner together with expected toner deposited on the image carrier. As a result, a mixture of toner of different colors is returned to the developing device. The mixture of toner would degrade the quality of the resulting color image, particularly color reproducibility, if again used to form a toner image on the image carrier.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-146334 and 9-288397.
It is therefore an object of the present invention to provide an image forming apparatus capable of obviating or effectively reducing the degradation of an image when the toner recycling system is applied to a tandem image forming apparatus.
In accordance with the present invention, an image forming apparatus includes at least two image carriers sequentially arranged in the direction of conveyance in which a recording medium is conveyed. Developing devices each form a toner image of a particular color on one image carrier. Cleaning devices each remove residual toner left on the surface of one image carrier after the transfer of the toner image to the recording medium. Toner recycling devices each return the residual toner removed from one image carrier by the cleaning device assigned thereto to one developing device that forms the toner image on the above image carrier. The toner recycling device is provided between one cleaning device, which removes the residual toner from the associated image carrier located at the most upstream position in the direction of conveyance, and one developing device assigned to the above image carrier, but not provided between another cleaning device, which removes the residual toner from another image carrier located at the most downstream position, and another developing device assigned to the above image carrier.
Also, in accordance with the present invention, an image forming apparatus includes at least two image carriers sequentially arranged in the direction of conveyance in which a recording medium is conveyed. Developing devices each form a toner image of a particular color on one image carrier. Cleaning devices each remove residual toner left on the surface of one image carrier after the transfer of the toner image to the recording medium. Toner recycling devices each return the residual toner removed from one image carriers by the cleaning device assigned to the image carrier to one developing device that forms the toner image on the above image carrier. The toner recycling device is present between one cleaning device, which removes the residual toner from the associated image carriers located at the most upstream position in the direction of conveyance, and one developing device assigned to the image carrier and between another cleaning device, which removes the residual toner from at least one image carrier other than the image carrier located at the most upstream position, and another developing device assigned to the image carrier. One of the developing devices, to which the residual toner is returned by the toner recycling device, located at the most downstream position, stores black toner for forming a black toner image.
Further, in accordance with the present invention, an image forming apparatus includes at least two image carriers sequentially arranged in the direction of conveyance in which a recording medium is conveyed. Developing devices each form a toner image of a particular color on one image carrier. A recording medium carrier conveys the recording medium. Cleaning devices each remove residual toner left on the surface of one image carrier after the transfer of the toner image to the recording medium. A cleaner removes toner deposited on the recording medium carrier. A toner recycling device is provided between at least one of the cleaning devices and one developing device associated with the cleaning device for returning the residual toner removed from one image carrier by the above cleaning device to the developing device. At a time other than a time for transferring toner images from the image carriers to the recording medium, a controller controls a mixed toner consumption mode which causes any one of the developing devices other than one located at the most upstream position in the direction of conveyance to deposit the toner on the associated image carrier, transfers the toner to the recording medium carrier, and causes the cleaner to remove the toner from the recording medium carrier.
Moreover, in accordance with the present invention, an image forming apparatus includes at least two image carriers sequentially arranged in the direction of conveyance in which a recording medium is conveyed. Developing devices each form a toner image of a particular color on one image carrier. Cleaning devices each remove residual toner left on the surface of one image carrier after the transfer of the toner image to the recording medium. Toner recycling devices each return the residual toner removed from one image carrier by the cleaning device assigned to the image carrier to one of the developing devices that forms the toner image on the above image carrier. The toner recycling devices each are provided between one of the cleaning devices and one developing device assigned to one image carrier, from which the cleaning device removes the residual toner. One of two developing devices adjoining each other in the direction of conveyance and located at the upstream position stores toner whose margin as to color mixture is smaller than a margin of toner stored in the other developing device located at the downstream position.